A stablecoin should be thought of as being tripartite: an entry, an exit, and circulation in between.

The exit has to come first in the order of operations. Ironically, many stables have limited exit. MakerDAO/Sky has the best one - permissionless swaps to USDC. Only improvement would be permissionless swaps to an external USD account.
The circulation is hard. Most stablecoins fall back on yield, and never get much further. Maybe it’s collateral. But rarely a settlement asset or unit of account.
I think most stables do badly here because they go wide and not deep. Crypto allows you to launch everywhere, all at once. So people do. And never develop that network of stores, bankers, borrowers, workers that accept the stablecoin.
Entry is surprisingly janky for most stables. It’s the famous problem that medieval mints had - if the coin is worth $1, then it takes $1 of material/collateral to make it. But minting isn’t free, and transport usually isn’t either. Both are cheaper for stablecoins than metallic coins, but you still need someone to bear the cost of minting + distribution.
Historically, this was done by paymasters who lacked physical coins, so they paid to mint them. Or by private lenders who offered banknotes to merchants in exchange for receivables at a discount. Or by banks lending notes secured by illiquid collateral like farmland.
So you mostly don’t have a smooth entry point for stablecoins that don’t lend or engage in quantitative easing, since you’re back to the medieval royal mint problem but probably don’t have someone looking to pay 1,000 workers in USD despite being short of physical currency.
Stablecoins that never gain moneyness skip that circulation step (or the only “circulation” is being nailed to someone’s EOA by compelling yield), which makes them essentially MMFs or managed investment funds.
But I think entry and exit are also underdeveloped for most stables - including USDT and USDC.